Michael Bloomberg has come under heavy fire at his first Democratic presidential debate, with rivals leaping to criticise him as a billionaire copy of US President Donald Trump who would lead the party to defeat in November.

Wednesday’s nationally televised debate was for many American voters the first unscripted look at Bloomberg, a media mogul and former New York mayor whose campaign until now has been fuelled by hundreds of millions of dollars of self-funded television ads and carefully choreographed personal appearances.

Bernie Sanders, Elizabeth Warren, Amy Klobuchar, Joe Biden and Pete Buttigieg lined up to attack Mr Bloomberg, accusing him of trying to buy his way into the White House and criticising his record on race and his history of sexist and misogynist comments.

Ms Warren, a senator from Massachusetts, led the pile-on on stage in Nevada

“We’re running against a billionaire who calls women fat broads and horse-face lesbians,” she said.

“No, I’m not talking about Donald Trump, I’m talking about Mayor Bloomberg.”

“Democrats take a huge risk if we just substitute one arrogant billionaire for another.”

Mr Bloomberg, who entered the race in November and is skipping the first four early voting states in February to focus on later nominating contests in March, said he did not inherit his money, but made it as a businessman.

“I’m spending that money to get rid of Donald Trump – the worst president we’ve ever had. And if I can get that done, it will be a great contribution to America and to my kids,” he said.

Mr Sanders criticised Mr Bloomberg’s support for “stop-and-frisk” police policies as mayor – which Mr Bloomberg has apologised for – that “went after African-American and Latino people in an outrageous way. That is not a way you are going to grow voter turnout”.

Mr Biden, the former vice president, said Mr Bloomberg had not managed New York very well during his three terms as mayor and said stop and frisk had thrown “close to five million young black men up against the wall”.

Elizabeth Warren: "Mike Bloomberg, drop out of the race. Keep putting your money in to help Democrats beat Donald Trump. And just take your ego out of it" #DemDebate https://t.co/MFtzpRBQNg pic.twitter.com/B11bg0Amcg

— CNN Politics (@CNNPolitics) February 20, 2020

Mr Bloomberg said he was “worried” and “embarrassed” about his support for stop and frisk and said he had apologised for supporting it.

“If we took off everybody that was wrong off this panel, everybody that was wrong on criminal justice sometime in their careers, there’d be nobody else up here,” Mr Bloomberg said.

But his rivals were not willing to let him off the hook.

“It’s not whether you apologise or not, it’s the policy. The policy was abhorrent. And it was in fact a violation of every right people have,” Mr Biden said.

The debate comes at a pivotal time, three days before Nevada’s presidential caucuses, the third contest in the state-by-state race to find a challenger to Mr Trump in the November 3 election.

Mr Biden and Ms Warren, in particular, face the do-or-die task of reigniting their campaigns after poor showings in Iowa and New Hampshire earlier this month.

Mr Bloomberg, 78, has come under heavy criticism on the campaign trail recently as his poll numbers have surged and his entry into the race on March 3 – known as Super Tuesday, when 14 states vote – draws closer.

He has risen to No.2 among Democrats behind Mr Sanders, according to a Reuters/Ipsos national poll released on Tuesday.

Mr Trump waded into the debate pile-on of Mr Bloomberg, telling supporters at a rally in Phoenix: “I hear he’s getting pounded tonight.”

-AAP